Metallurgical silicon containing phosphorus for the preparation of organohalogenosilanes

ABSTRACT

A metallurgical silicon is disclosed which contains substantially from 30 to 180 ppm of phosphorus comprised of a crystalline phase of primary silicon having a dissolved phosphorus content between 30 and 150 parts per million and intermetal phases whose global ratio is between 0.5 and 2%, wherein among the intermediary phases, the phase Si 2  Al 2  Ca does not exceed 0.3% of the total of the metallurgical silicon mass. The metallurgical silcon is used for the synthesis of alkyl or aryl halogenosilanes useful for the preparation of silicones.

FIELD OF THE INVENTION

The innovation relates to a particular type of metallurgical siliconhaving a controlled structure and containing a controlled quantity ofphosphorus whose distribution in the various phases which constitute thestructure of the metallurgical silicon is itself controlled.

This type is particularly well adapted to the synthesis reaction ofalkyl or aryl halogenosilanes.

DESCRIPTION OF RELATED ART

The synthesis of alkyl or aryl halogenosilanes (which will hereinafterbe called "silanes") through the reaction of a halogenated hydrocarbonwith silicon in the presence of a copper-based catalyst at a temperatureof 250° to 350° C. is known from U.S. Pat. No. 2,380,995 granted toRochow on Aug. 7, 1945.

This reaction, hereinafter referred to as "the Rochow reaction", hasundergone considerable industrial development, having been in particularthe basis for the entire silicone industry.

This reaction is generally produced with methyl chloride CH₃ Cl andresults in a mixture of various methyl chlorosilanes, particularlymonomethyltrichlorosilane CH₃ SiCl₃ (which will hereinafter be called T)and dimethldichlorosilane (CH₃)₂ SiCl₂ (which will hereinafter be calledD). Since the most sought-after product is D, it is very important to beable to control the reaction in such a way as to obtain the maximumproportion of D in the mixture of silanes obtained, a ratio which iscalled selectivity. It is also very important to be able to produce themaximum quantity of silanes per unit of time, the value of the weightflow of silanes produced being called reactivity.

Since the initial Rochow patent of 1945, a vast amount of work has beendevoted to increasing the reactivity and selectivity of the Rochowreaction. Research had been conducted on the technology of theprocess--which, while originally carried out in a fixed bed, is nowalmost always done in a fluidized bed--on the physical form of thesilicon used (particle size distribution, etc . . .) and particularly onthe catalytic systems used, and on the chemical composition of thesilicon, this silicon being an industrial product which contains acertain number of impurities.

U.S. Pat. No. 4,602,101 granted to Halm et al on Jul. 22, 1986, provides(columns 1 through 4) a fairly extensive summary of the knowledge of thecatalytic systems studied. This summary shows that a certain number ofelements or compounds have been described as catalysts of the Rochowreaction, while others have been indicated as cocatalysts or catalystpromoters because their use in conjunction with certain catalysts makesit possible to improve selectivity and/or reactivity.

The original catalyst, copper, is the most often cited and is utilizeduniversally. However, nickel, antimony, manganese, silver, titanium andiron are also cited as catalysts. Copper can be used either in metallicform or in the form of oxides (possibly formed in situ from otherderivatives such as copper nitrate), or in the form of halides.

It is possible to add promoters to these catalysts, or to some of them,which can either be elements such as:

zinc, cadmium, mercury;

tin, particularly in the presence of copper or copper and zinc;

the metals from group VII of the periodic table of the elements;

phosphorus or certain phosphorus-containing derivatives; or compoundssuch as:

copper oxides or other catalyst metals;

metal hydroxides from group IV of the periodic table of the elements,used with copper;

refractory hydrous oxides, for example hydrous aluminas, used withcopper;

copper, iron, or zinc halides or other copper salts (formate), used withcopper and/or its oxides, and sometimes with iron.

Some of these catalysts and promoters are particularly noteworthy,specifically copper as a catalyst and zinc, tin and phosphorus aspromoters.

Since Jan. 15, 1954, the publication date of U.S. Pat. No. 2,666,776(Nitzsche et al.), the beneficial influence of phosphorus as acocatalyst or promoter has been recognized: column 1, lines 26-37,indicates that alkyl or aryl halides of silicon, particularly dialkyl ordiarylhalogenosilanes, are obtained by using an alloy which contains, inaddition to silicon and copper, a metal from the 5th or 8th group of theperiodic table, particularly cobalt, nickel, iron or phosphorus, andthat an additional increase in effectiveness is obtained if the catalystis used in connection with an activation agent, for example a coppersalt.

The importance of phosphorus was confirmed, first by Rossmy in hisGerman patent 1 165 026 filed Sep. 14, 1959 and granted Mar. 12, 1964,and later by Trofimova in the Soviet application No. 754 859 filed Dec.8, 1961, which resulted in the granting of the certificate of inventionNo. 157 349, said certificate being mentioned in the February, 1966,issue of "Soviet Invention Illustrated." This certificate mentions theutilization of a silicon-copper alloy which also contains antimony andphosphorus. In an example, it specifies a copper content of 10%, anantimony content of 40 ppm, and a phosphorus content of 200 ppm.

In an article published in Zhurnal Obshchei Khimii, Vol. 34, No. 8, pp.2 706 through 2 708 (August 1964), Lobusevich, Trofimova et al. showthat phosphorus used alone is a reaction poison in the presence of acopper catalyst, but that conversely, it results in an improvement inselectivity in the presence of other promoters, such as antimony,arsenic, zinc. This positive effect is optimal for a phosphorus contentbetween 100 and 200 ppm.

Since these publications, there has been an effort to use phosphorus asa promoter in the Rochow reaction, and various formulas and techniqueshave been described, for example:

Ward, in his U.S. Pat. No. 4,500,724, describes the utilization ofcomplex formulas comprising copper (or copper chloride), zinc, tin andphosphorus, which produce particularly advantageous results with regardto reactivity and selectivity;

Halm, in his above-mentioned U.S. Pat. No. 4,602,101, describes theutilization in the Rochow reaction of a promoter chosen from amongelementary phosphorus, metallic phosphides, or compounds capable offorming metallic phosphides under the conditions of the reaction, in aquantity of 25 to 2,500 ppm relative to the silicon, in the presence oftin and copper;

For economic reasons, the silicon generally used in the Rochow reactionis a metallurgical silicon produced through carboreduction of silica inan electric furnace, then refined in order to adjust the ratio of itsprincipal impurities, and finally cast into loaves and solidified. Thesolid mass is then reduced to a powder with a particle size distributionadapted for utilization in industrial silane production facilities.There are numerous monographs which relate to this technique of siliconproduction, one of which, for example, may be found in Chapter II,"Silicon Alloys," of the work by Elyutin et al., "Production ofFerroalloys-Electrometallurgy," published in English in Moscow in 1957by "The State Scientific and Technical Publishing House for Literatureon Ferrous and Non Ferrous Metallurgy."

This metallurgical silicon contains a certain number of principalimpurities, essentially calcium, aluminum, and iron, all of which arepresent in proportions between 0.01 and 1%, proportions which areadjusted during the refining process in order to meet the specificationsrequired by the market. It also contains secondary impurities containedin the raw materials, which the process of production and refining doesnot always make it possible to eliminate. These impurities, as well asthe proportions of their presence in the silicon, generally between 10and 500 ppm, greatly depends on the origin of the raw materials used.The most common of these impurities are metalloids, phosphorus, boron,sulfur, carbon, or metals, titanium, copper, magnesium, manganese,nickel, vanadium, zirconium, etc.

The source of the phosphorus content, as revealed in Chapter II of theabove-mentioned work by Elyutin, pp. 65 through 68, is the various rawmaterials used in the production of metallurgical silicon, and it isshown in particular how the phosphorus content of the silicon producedcan be determined from that of the various raw materials. The phosphoruscontents usually encountered vary between 25-30 ppm and 400-500 ppm. Thearticle "High-Purity Silicon for Solar Cell Application" by Dosaj et al.published by the Journal of Metals, June, 1978, in its Table VII on p.12, gives a value of 50 ppm, the article "Efficient PolycrystallineSolar Cells Made from Low-Cost Refined Metallurgical Silicon," by Hanekaet al., Thirteenth IEEE Photovoltaic Specialists Conference-1978, pages485-9, published by the IEEE, New York, in 1978, in its Table I on p.487, indicates a value of 100 ppm, and the above-mentioned work byElyutin, on page 67, indicates a value of 400 ppm.

It is therefore absolutely certain that since the beginning of silaneproduction, the phosphorus present in the silicon has made acontribution to the catalysis of the Rochow reaction, a variable andoften random contribution depending on the phosphorus content of thesilicon used, but an effective contribution for all silane producers.

Recently, Dosaj, Halm and Wilding obtained European patent 0 272 860,granted Oct. 7, 1992, while Halm and Wilding obtained European patent 0273 635, granted Feb. 10, 1993. These two patents describe theutilization in the Rochow reaction of a silicon which contains from 25to 2,500 ppm of a phosphorus-containing promoter in the presence of acatalyst which contains copper and tin, and possibly zinc. In Europeanpatent 0 272 860, this promoter is introduced into the silicon bychoosing and metering raw materials in accordance with the methoddescribed in Chapter II, p. 65 through 68, of the above-mentioned workby Elyutin, whereas in European patent 0 273 635, the promoter is simplya nonvolatile phosphorus-containing compound added to the liquid siliconduring its refining.

However, these techniques are not satisfactory, for it has becomeapparent that two samples of silicon containing the same quantity ofphosphorus obtained through either one of these techniques could causesubstantially different modifications in the selectivity and reactivityof the Rochow reaction, as will be shown below.

SUMMARY OF THE INVENTION

During research intended to eliminate or at least reduce thesenon-reproducible results, Applicant have found that metallurgicalsilicon was constituted by various phases, the principal one of which isa very pure crystallized silicon phase, referred to below as primarysilicon, in which only two dissolved impurities, boron and phosphorus,can exist, the other phases being constituted by various combinations ofsilicon and principal and secondary impurities.

Applicant have also found that the phosphorus present in metallurgicalsilicon was distributed among the various phases present, particularlybetween the dominant phase of primary silicon and, when it exists, aternary Si₂ Al₂ Ca phase.

The invention relates to a type of metallurgical silicon which isparticularly well adapted to the Rochow reaction with its improvedcharacteristics of selectivity and reactivity, characterized by thepresence of 30 to 150 ppm phosphorus in the dominant phase of primarysilicon and by the absence or near-absence of a ternary Si₂ Al₂ Ca phasewhose quantity relative to the total mass of metallurgic silicon mustnot exceed 0.3%.

The invention also relates to a process for preparing a product of thistype.

DETAILED DESCRIPTION OF THE INVENTION

The various intermetallic phases which can exist in metallurgicalsilicon in addition to the dominant phase of primary silicon weredescribed by Margaria et al. in "Proceedings of the 6th InternationalFerroalloys Congress Cape Town," Vol. 1, Johannesburg, SAIMM, 1992, pp.209 through 214. There are seven principal phases to which secondaryphases are added which depend on the secondary impurities which areeffectively present, only one of which (Si₂ Al₂ Ca) is capable ofincluding appreciable quantities of phosphorus, as shown by Tables I andII, p. 210. This article shows how these intermetallic phases arecreated during the solidification process of the metallurgical silicon,and how they are deposited at the grain boundaries of the dominantphase. Also, on p. 210, it provides a means of analysis by which, usingimages from electron microscopy and X-ray diffraction, it is possible todiscover the spatial distribution of the various phases and theircomposition.

These same techniques, combined with a complex mathematical process, canbe used to evaluate the respective ratio of each of the phases in themetallurgical silicon studied. Generally, the aggregate proportion ofall the intermetallic phases is between 0.5 and 2% and essentiallydepends on the proportions of the principal impurities which areadjusted during the refining process.

The total quantity of phosphorus introduced into the metallurgicalsilicon during its preparation, whether through the raw materials orthrough any other addition, are distributed, as indicated in theabove-mentioned article by Margaria, between the dominant phase and theSi₂ Al₂ Ca phase, the other intermetallic phases not being able toaccept significant fractions of it. Thus, it was also found, forexample, that in a metallurgical silicon which contained 200 ppmphosphorus total, the dominant phase of primary silicon would onlycontain 50 ppm phosphorus, while the Si₂ Al₂ Ca phase, present at aproportion of 1.5%, would contain 10,000 ppm phosphorus.

Applicants have found that, surprisingly, the quantities of phosphoruscontained in the dominant phase and in the Si₂ Al₂ Ca phase did not havethe same effect on the modification of the selectivity and reactivity ofthe Rochow reaction. The silicon contained in the dominant phase ofprimary silicon has a beneficial effect on selectivity and reactivity aslong as its ratio is between 30 and 150 ppm, its effect beinginsignificant below 30 ppm and becoming harmful, decreasing selectivity,as the ratio increases beyond 150 ppm. These results are comparable tothose obtained by Lobusevich and Trofimova in the above-mentionedarticle, the phosphorus in that case apparently having been added at themoment of the reaction rather than being added to the silicon used.

The phosphorus contained in the Si₂ Al₂ Ca phase, on the other hand,does not have a positive effect by itself. On the contrary, itcontributes to a degradation of the selectivity, which actually seems todepend on the total quantity of phosphorus contained in themetallurgical silicon.

In summary, metallurgical silicon with the same aggregate chemicalcomposition, particularly where its proportion of phosphorus isconcerned, will produce substantially different results in the Rochowreaction, depending on the distribution of the phosphorus it containsbetween the dominant phase of primary silicon and the intermetallic Si₂Al₂ Ca phase, the other intermetallic phases having no notable effect.

The silicon which leads to the best results in the Rochow reaction is asilicon containing between 30 and 180 ppm phosphorus. Almost all of thisphosphorus is contained in the dominant phase of primary silicon. Therest, namely a maximum of about 30 ppm, is linked to the Si₂ Al₂ Caphase, which can contain 10,000 ppm but in which its content is limitedto 0.3% of the weight of the silicon. This result can be obtained bycontrolling the total quantity of phosphorus introduced into themetallurgical silicon through its raw materials, as well as the quantitywhich may specifically be added to the liquid silicon during itsproduction, its furnace casting, its refining, or its final casting, andalso by controlling the formation of the Si₂ Al₂ Ca phase so as to limitit to a maximum of less than 0.3%, for example, and preferably less than0.1%.

To obtain this type of silicon, it is possible to use the usual processfor producing metallurgical silicon through the carboreduction of silicain an electric furnace, the raw materials being chosen by the methoddescribed in the above-mentioned work by Elyutin et al. in order toendow the product obtained with a specific total proportion ofphosphorus, a proportion which is capable of being adjusted to thedesired value between 30 and 180 ppm by adding a nonvolatile compound ofphosphorus, such as tricalcium phosphate to the liquid silicon at anymoment.

This silicon is then refined in the liquid phase by means of a standardtreatment with the aid of an oxidizing agent, for example oxygen or air,which partially eliminates some of the most oxidizable impurities suchas aluminum and calcium, the majority of the impurities, particularlythe phosphorus, being unaffected and remaining in the liquid phase.

It is then cast in the form of ingots which, as seen above, include adominant phase of primary silicon containing as impurities onlyphosphorus and boron and various intermetallic phases situated at thegrain boundary of the dominant phase.

To control the presence of the Si₂ Al₂ Ca phase and to limit it to 0.3%,preferably 0.1%, it is possible to simply cool the cast mass veryslowly, particularly when passing through the sensitive zone from 1200°C. to 800° C., as indicated in the above-mentioned article by Margaria,page 213. However, this technique has industrial disadvantages due toits slowness and to the fact that in a massive ingot it results in astructure that is too heterogeneous.

In order to obtain the desired structure (limitation of the Si₂ Al₂ Caphase to 0.3% and preferably to 0.1%) throughout the solidified massusing industrial solidification processes, two conditions will besatisfied simultaneously: one of these consists of adjusting the ratioof the impurities Fe, Al, Ca in the silicon to within a very preciserange; the other consists of determining the conditions forsolidification. In general, the range of composition is chosen as afunction of the solidification speeds which are effectively realized.

The desired result can thus be obtained by simultaneously adjusting twoweight ratios:

--the ratio of the sum of the weight percentages of aluminum and calciumto the weight percentage of iron in the silicon, (Al+Ca)/Fe. This ratiowould have to be between 0.7 and 0.9 for slow solidification speeds,that is, such that the temperature is reduced from 1000° C. to 800° C.at a rate between 6° and 30° C./min. This ratio would have to be between0.5 and 0.7 for rapid solidification speeds, that is, such that thetemperature is reduced from 1000° C. to 800° C. at a rate between 30°and 120° C./min.

--the ratio of the weight percentage of aluminum to that of calcium inthe silicon, Al/Ca, which would have to be between 2.5 and 4.5, andpreferably between 3.3 and 3.7.

This adjustment of these two weight ratios can advantageously be carriedout during the oxidizing refining of the silicon, which consists ofinjecting air and/or oxygen into the molten silicon. This refiningreduces the calcium and aluminum contents by adjusting the quantity andthe duration of action of the oxidizing agent until the desired weightratios are obtained and if necessary, by adding more of either of thesetwo elements if its elimination has been excessive.

The silicon whose characteristics and production process has beendescribed above finds application in the direct, so-called Rochowreaction for producing alkyl or aryl halogenosilanes through a reactionwith alkyl or aryl halides at a temperature between 250° and 350° C., inthe presence of a copper-containing catalyst and possibly one or morepromoters: tin, zinc, antimony.

EXAMPLES

Example 1

Production of an Si which has an excessive Si₂ Al₂ Ca content.

The silicon is produced through carbothermy in a reduction furnace fromsilicas and reducing agents: coals, cokes, woods, charcoals. Thisoriginal silicon, including the raw materials used, contains 0.28% iron,0.7% Ca, 0.6% aluminum and 90 ppm phosphorus.

This silicon is then subjected to an oxidizing refining in a ladlethrough an addition of silica and an injection of air and/or oxygenintended to reduce the calcium and aluminum contents. Thus a silicon isobtained which contains 0.28% iron, 0.080% calcium, 0.12% Al. Thephosphorus not affected by the refining retains a content of 90 ppm.Next, 1.6 kg of aluminum and 0.5 kg of a CaSi alloy having 30% calciumis added per ton of liquid Si. The liquid alloy, homogenized by aninsufflation of nitrogen, is then cast and solidified into ingots with athickness of 10 cm on the one hand, and ingots with a thickness of 20 cmon the other hand, in cast iron ingot molds.

After a representative sampling of the solid silicon, an analysis showsthat this silicon contains 0.31% iron, 0.26% aluminum, 0.09% calcium, 90ppm phosphorus.

According to these analyses, on the one hand, (Al+Ca)/Fe=1.13 andAl/Ca=2.8.

A ground section of the silicon thus obtained is examined with the aidof a scanning electron microscope connected to an image analysis system.

Thus it is possible to identify the Si₂ Al₂ Ca phase from among thevarious intermetallic phases and to determine its proportion.

The results of these tests show that the Si₂ Al₂ Ca phase has a totalcontent of 0.15% which can reach 0.3% locally in the silicon cast iningots with a thickness of 20 Cm, and has a total content of 0.2% whichcan reach 0.4% locally in the silicon cast in ingots with a thickness of10 cm.

The microanalyses carried out with the aid of an electron microprobeshow that the P content of the Si₂ Al₂ Ca phase reaches 1.2% and thatthe primary silicon crystal has phosphorus contents which vary from 50to 70 ppm for silicon cast in ingots with a thickness of 20 cm and whichvary from 40 to 60 ppm for silicon cast in ingots with a thickness of 10cm.

Example 2

Production of a silicon according to the invention which contains littleSi₂ Al₂ Ca.

A silicon containing 0.35% iron, 0.05% calcium, 0.12% Al is produced ina reduction furnace with subsequent refining as described above. Next,1.4 kg of aluminum and 0.6 kg of a 30% Ca CaSi alloy is added per ton ofliquid silicon. The liquid alloy, homogenized by an insufflation ofnitrogen, is then cast and solidified into ingots with thicknesses of 10and 20 cm in cast iron ingot molds.

After a representative sampling of the solid silicon, an analysis showsthat this silicon contains 0.35% iron, 0.25% aluminum, 0.06% calcium, 90ppm phosphorus.

According to these analyses, on the one hand, (Al+Ca)/Fe=0.88, andAl/Ca=4.2.

The tests described above show that the Si₂ Al₂ Ca phase is absent fromthe silicon cast into ingots with a thickness of 20 cm and has a totalcontent of 0.1% which can reach 0.2% locally in the silicon cast intoingots with a thickness of 10 cm.

Microanalyses of the silicon show that the crystal of primary siliconhas phosphorus contents of 90 ppm for the silicon cast in ingots with athickness of 20 cm and contents which vary from 55 to 70 ppm for thesilicon cast in ingots with a thickness of 10 cm.

Example 3

Production of a silicon according to the invention which does notcontain any Si₂ Al₂ Ca.

A silicon containing 0.37% iron, 0.05% calcium, 0.10% Al is produced ina reduction furnace with a subsequent refining as described above. Next,1 kg of aluminum and 0.6 kg of a 30% Ca CaSi alloy is added per ton ofliquid silicon. The liquid alloy, homogenized by an insufflation ofnitrogen, is then cast and solidified into ingots with thicknesses of 10and 20 cm in cast iron ingot molds.

After a representative sampling of the solid silicon, an analysis showsthat this silicon contains 0.37% iron, 0.19% aluminum, 0.06% calcium and90 ppm phosphorus.

According to these analyses, on the one hand, (Al+Ca)/Fe=0.67 andAl/Ca=3.2, values which conform to the recommendations of the presentinvention.

The same test as that conducted in the two preceding examples did notdetect the presence of a Si₂ Al₂ Ca phase, either in the silicon castinto ingots with a thickness of 20 cm or in the silicon cast into ingotswith a thickness of 10 cm.

The microanalyses of the silicon show that the crystals of primarysilicon contain from 85 to 90 ppm phosphorus in both cases: therefore,the totality of the primary silicon effectively contains the desiredconcentration of phosphorus.

Example 4

This example shows the advantage of the presence of a sufficientquantity of phosphorus in the primary phase, obtained through theabsence of a Si₂ Al₂ Ca phase, in the synthesis of methylchlorosilanes.

Two samples of silicon 1 and 2 having the same phosphorus content - - -30 parts per million - - - were compared; the analyses were thefollowing:

    ______________________________________                                        Sample          1       2                                                     ______________________________________                                        Fe (%)          0.37    0.41                                                  Al (%)          0.4     0.17                                                  Ca (%)          0.18    0.054                                                 Ti (%)          0.024   0.028                                                 P (ppm)         30      30                                                    (Al + Ca)/Fe    1.57    0.55                                                  Al/Ca           2.22    3.15                                                  ______________________________________                                    

Sample 1, according to its analysis, contains 0.36% Si₂ Al₂ Ca; theprimary silicon contains only 5 to 10 ppm phosphorus and the sample istherefore not in conformity with the invention.

Sample 2, by reason of its analysis and its mode of cooling, is inconformity with the invention: it does not contain any Si₂ Al₂ Ca andthe concentration of phosphorus in the primary silicon is equal to thetotal content, namely 30 ppm.

Both of these samples were subjected to a methylchlorosilane productiontest under the following conditions:

The tests were conducted in a fluidized bed in a glass reaction vesselwith a diameter of 30 mm equipped with an agitator. The same quantity ofsilicon having the same particle size distribution, between 71 and 160μm, was used in each test. The reaction mixture contained 40 g ofsilicon, 3.2 g of partially oxidized copper as a catalyst, and 0.05 g ofZnO.

Methyl chloride was added to the reaction mixture through a sinteredglass disk at a pressure of 2 bars. The quantity of methyl chloride waskept constant at 1.8 liters/h, measured at a pressure of 2 bars. Afterthe reaction medium was heated and the reaction started, the temperatureof the system was adjusted and maintained at 300° C. and the quantityand composition of the silane mixture formed was determined. The valuesindicated in the table below are the averages of 4 individualmeasurements.

In this table, P indicates the quantity of silanes produced in g/hour;MeH, Mono, T, D, PS indicate the respective percentages by weight ofmonomethyldichlorosilane (CH₃ HSiCl₂), trimethylchlorosilane ((CH₃)₃SiCl), methyltrichlorosilane (CH₃ SiCl₃), dimethyldichlorosilane ((CH₃)₂SiCl₂) and finally polysilanes. Since dimethyldichlorosilane is thedesired product, the selectivity is estimated from the value of D, whichmust be as high as possible, and from that of T/D, which must be as lowas possible.

    ______________________________________                                        Smpl.                                                                              P (g/h) MeH (%)  Mono (%)                                                                             T (%)                                                                              D (%) T/D  PS (%)                           ______________________________________                                        1    7.2     2.7      3.2    7.3  86.4  0.084                                                                              4.9                              2    6.8     2.0      2.5    4.9  90.3  0.054                                                                              5.2                              ______________________________________                                    

The gain in selectivity obtained with alloy 2 according to the inventionis clear.

Example 5

This example shows that the results obtained in the synthesis ofmethylchlorosilane are similar, despite different total phosphoruscontents, as long as these contents are similar in the primary siliconphase.

Two samples of silicon 3 and 4 having different phosphoruscontents - - - 80 and 30 ppm - - - were compared, the analyses of whichwere the following:

    ______________________________________                                        Sample          3       4                                                     ______________________________________                                        Fe (%)          0.29    0.41                                                  Al (%)          0.32    0.17                                                  Ca (%)          0.17    0.054                                                 Ti (%)          0.019   0.028                                                 P (ppm)         80      30                                                    (Al + Ca)/Fe    1.69    0.55                                                  Al/Ca           1.88    3.15                                                  ______________________________________                                    

Sample 3, according to its analysis, contains 0.31% Si₂ Al₂ Ca; theprimary silicon contains 40 ppm phosphorus and the sample is thereforenot in conformity with the invention.

Sample 4, by reason of its analysis and its mode of cooling, is inconformity with the invention: it does not contain any Si₂ Al₂ Ca andthe concentration of phosphorus in the primary silicon is equal to thetotal content, namely 30 ppm, very near that of Sample 3.

Both of these samples were subjected to the same methylchlorosilaneproduction test described in Example 4.

The results appear in the table below.

    ______________________________________                                        Smpl.                                                                              P (g/h) MeH (%)  Mono (%)                                                                             T (%)                                                                              D (%) T/D  PS (%)                           ______________________________________                                        3    6.1     1.8      2.3    5.4  90.3  0.059                                                                              3.6                              4    6.8     2.0      2.5    4.9  90.3  0.054                                                                              5.2                              ______________________________________                                    

It is noted that the selectivity performances obtained are similar toone another, which is due to the fact that the phosphorus content of theprimary silicon is similar in both samples. On the other hand, it isnoted that there is a slight degradation of reactivity in the samplehaving the higher total phosphorus content.

Example 6

This example allows a comparison of Samples 5, 6, 7, 8 in which thereare successively higher phosphorus contents in the primary silicon. Thefirst three contain the primary Si₂ Al₂ Ca phase, the fourth does not.

The total phosphorus (Ptot) and the phosphorus content of the primarysilicon (Pprim) were determined.

The analyses were the following:

    ______________________________________                                        Sample     5       6          7     8                                         ______________________________________                                        Fe (%)     0.37    0.29       0.29  0.38                                      Al (%)     0.4     0.3        0.32  0.15                                      Ca (%)     0.18    0.16       0.17  0.056                                     Ti (%)     0.024   0.019      0.019 0.022                                     Ptot (ppm) 30      50         80    90                                        Pprim (ppm)                                                                              10      30         50    90                                        (Al + Ca)/Fe                                                                             1.57    1.59       1.69  0.54                                      Al/Ca      2.22    1.875      1.88  2.68                                      ______________________________________                                    

Samples 5 through 7 are not in conformity with the invention. OnlySample 8 is.

Each of these samples was subjected to the same methylchlorosilaneproduction test described in Example 4.

The results appear in the table below.

    ______________________________________                                        Smpl.                                                                              P (g/h) MeH (%)  Mono (%)                                                                             T (%)                                                                              D (%) T/D  PS (%)                           ______________________________________                                        5    7.2     2.7      3.2    7.3  86.4  0.084                                                                              4.9                              6    6.0     2.3      2.3    5.2  89.8  0.057                                                                              2.8                              7    6.1     1.8      2.3    5.4  90.3  0.059                                                                              3.6                              8    5.4     1.9      1.0    3.7  93.2  0.039                                                                              2.7                              ______________________________________                                    

It is clear that the selectivity performances obtained are particularlygood in Sample 8 according to the invention. It is noted that Samples 7and 8, in which the total phosphorus contents are similar, performdifferently, which is linked to the different phosphorus contents intheir primary Si phases.

We claim:
 1. A metallurgical silicon comprising a crystalline phase ofprimary silicon having a dissolved phosphorus content between 30 and 150ppm by weight and intermetallic phases in an amount of 0.5 to 2% byweight, said metallurgical silicon containing from 30 to 180 ppm byweight total phosphorus and not more than 0.3% by weight of anintermetallic Si₂ Al₂ a phase,said silicon containing Al and Ca in aweight ratio Al/Ca which is between 2.4 and 4.5.
 2. Silicon according toclaim 1, wherein said metallurgical silicon contains not more than 0.1%by weight of said intermetallic Si₂ Al₂ Ca phase.
 3. A metallurgicalsilicon comprising a crystalline phase of primary silicon having adissolved phosphorus content between 30 and 150 ppm by weight andintermetallic phases in an amount of 0.5 to 2% by weight, saidmetallurgical silicon containing from 30 to 180 ppm by weight totalphosphorus and not more than 0.3% by weight of an intermetallic Si₂ Al₂Ca phase,said silicon containing Al, Ca and Fe in a weight ratio(Al+Ca)/Fe which is between 0.5 and 0.9 and in a weight ratio Al/Cawhich is between 2.4 and 4.5.
 4. Silicon according to claim 3, wherein(Al+Ca)/Fe is between 0.5 and 0.7.
 5. Silicon according to claim 3,wherein (Al+Ca)/Fe is between 0.7 and 0.9.
 6. Silicon according to claim3, wherein Al/Ca is between 3.3 and 3.7.
 7. A process for thepreparation of metallurgical silicon comprising a crystalline phase ofprimary silicon having a dissolved phosphorus content between 30 and 150ppm by weight and intermetallic phases in an amount of 0.5 to 2% byweight, said metallurgical silicon containing from 30 to 180 ppm byweight total phosphorus and not more than 0.3% by weight of anintermetallic Si₂ Al₂ Ca phase, comprising the steps of:a) adjusting theamount of phosphorus in raw silicon in the liquid state to 30 to 180 ppmby weight; and b) solidifying the liquid raw silicon to solidmetallurgical silicon either at a slow solidification speed of between 6and 30° C./min between 1000° and 800° C., or at a rapid solidificationspeed of between 30° and 120° C./min between 1000° and 800° C., andlimiting the formation of an intermetallic Si₂ Al₂ Ca phase in thesolidified silicon by1) adjusting amounts of aluminum, calcium, and ironin the raw silicon such that the weight ratio (Al+Ca)/Fe is between 0.7and 0.9 for a slow solidification speed, or between 0.5 and 0.7 for arapid solidification speed; and 2) adjusting amounts of aluminum andcalcium in the raw silicon such that the weight ratio Al/Ca is between2.5 and 4.5.
 8. Process according to claim 7, wherein aluminum andcalcium are adjusted such that Al/Ca is between 3.3 and 3.7.